I hope this helped.
You have received great advice below. I would add that you can make your own drawing, it doesn't have to be to scale, however, must have accurate dimensions.
If you need a contractor to help with the expansion, I can give you a reference for very professional and reliable company.
They may ask you to submit a drawing of what you intend to do.
The building department will determine by your lot and set backs what you can build per zoning. As for a real estate determination of a garage, a two car garage is about 400 square feet, 20 x 20 (even though today's cars rarely fit in them. So a 10 x 20 or what you are proposing 12 x 20 probably could be counted as a single car garage.
Which ever you decide please get a permit and have it finalized. It will make resale for you and financing for the future borrower that much easier.
Have an amazing day!